Diagnostic apparatus for internal combustion engines has necessarily included means for making electrical and mechanical connections to an engine to measure engine speed, ignition timing, compression, etc. Now, it has been found possible to measure engine speed without making any electrical or mechanical connections to the engine by means including a transducer for sensing pressure variations at the end of the engine exhaust pipe due to explosions in individual cylinders of the engine. Such an apparatus is described in a patent application Ser. No. 573,032 filed on Apr. 30, 1975, by S. C. Hadden et al. for a "Tachometer Without Physical Connection to Internal Combustion Engine." The described apparatus for measuring engine speed includes means for tracking rapid changes in engine speed and, therefore, it can be used in the performance of an acceleration burst test for determining the torque and horsepower of the engine. The ability of the described apparatus to track rapidly changing engine speed is improved by inclusion of a signal-frequency tracking band-pass filter as described in a patent application, Ser. No. 573,034, filed on Apr. 30, 1975, by E. M. Sutphin, Jr, on a "Filter Which Tracks Changing Frequency of Input Signal." It is desirable to use these background techniques in a system for providing engine diagnostic information which is in addition to the torque and horsepower information provided by the described apparatus.